The main characteristic of diabetes is hyperglycemia. Chronic hyperglycemia induces severe complications of diabetes: retinopathy, cataracts, peripheral neuropathy, nephropathy, and vascular angiopathy. It is also a major health problem such that the rate of morbidity and mortality of diabetes is third greatest after cancer and cardiovascular disease. It is important to note that diabetic patients have a much higher incidence of cardiovascular disease than non-diabetics. It was proven that the better the control of blood glucose, the lesser the complications of diabetes. The main acute complication in type 1 diabetes is hypoglycemia. This problem has been greatly enhanced by introduction of tight glucose control. Because of the threat of hypoglycemia, many patients will relax their glucose control in order to minimize the problem of hypoglycemia, which then increases the threat of chronic complications. Thus, hypoglycemia is the limiting factor in the treatment of type 1 diabetes. In addition, many non-diabetic subjects suffer from episodes of hypoglycemia of unknown etiology. One of the main problems in diabetic subjects is defective counterregulation (mainly glucagon, epinephrine, norepinephrine, and cortisol responses) to hypoglycemia.
The initial abnormal counterregulatory response in diabetes (1) is diminished glucagon responsiveness. This is paradoxical because the glucagon response to neurogenic stress (2;3) and exercise (4-7) is normal. One explanation for the discrepancy between hypoglycemia and other stress responses is that the α-cell becomes more sensitive to the inhibitory effect of insulin in diabetes because in type 1 diabetes, most β-cells are destroyed. The sensitivity of α-cells during hypoglycemia improves when normoglycemia is achieved by chronic phloridzin, but not by insulin treatment in diabetic rats (1). This is not surprising because it is well known that insulin inhibits glucagon synthesis and release (8). It is suggested that insulin released from β-cells acts directly on α-cells. It is known that α-cells have insulin receptors. When insulin binds to those receptors, a cytosolic receptor (GABAAR) is translocated to the cell membrane. This induces membrane depolarization and consequently, glucagon secretion is suppressed (9;10). When hypoglycemia is induced with insulin in clinical investigations in non-diabetic subjects, glucagon secretion promptly increases and consequently, normal blood glucose is restored. This occurs because low blood glucose by itself increases glucagon release, and this effect is stronger than the inhibitory action of insulin on α-cells. In contrast, in type 1 diabetic subjects, there are very few or no β-cells in the pancreas, and therefore α-cells become sensitized to insulin. Under those conditions, the effect of an increased amount of insulin in the blood is much stronger than the effect of low blood glucose. Therefore, in diabetic patients which are insulin treated, insulin's effect on the α-cell is much stronger than the effect of blood glucose, and consequently, during hypoglycemia, the glucagon response is either greatly decreased or absent.
One additional possibility for the increased α-cell sensitivity to insulin is the augmented amount of somatostatin in the pancreas in diabetic animals (11;12) as well as in diabetic humans (13). In streptozotocin (STZ)-diabetic rats, there is: (1) hyperplasia and hypertrophy of somatostatin-containing δ-cells in the pancreas (13), (2) increased expression of pancreatic prosomatostatin mRNA (14;15), (3) increased pancreatic somatostatin (1), (4) distribution of somatostatin-secreting δ-cells in the central portions of islets cells (16).
The present inventors were the first to suggest 17 years ago that excessive somatostatin may inhibit glucagon release during hypoglycemia (11). It is well documented that somatostatin inhibits stimulated secretion of pancreatic glucagon.
In STZ-diabetic rats, the expression of the gene for pro-glucagon and pro-somatostatin are both markedly increased (15). This increased concentration of somatostatin is observed in diabetic rats, both during euglycemia (i.e. normal blood glucose concentrations) and hypoglycemia (1). Concentration of somatostatin in plasma is also increased during euglycemia and hypoglycemia in diabetic rats (1). However, despite increased gene expression of proglucagon, plasma concentrations of glucagon are not increased during hypoglycemia in diabetic rats, presumably in part due to the marked elevation of somatostatin levels.
Somatostatin receptors are ubiquitously expressed in most tissues of the body. So far, 5 different subtypes of somatostatin receptors have been discovered. It is not desirable to inhibit all somatostatin receptors, which may cause unfavourable side effects. The localization of particular receptor subtypes on different tissues allows for specific receptor antagonists to exert specific inhibitory effects. For protection against hypoglycemia, the most important is inhibition of somatostatin receptors related to counterregulatory hormone release which are found in the pancreas, adrenal gland, and hypothalamus of the brain. Somatostatin receptor type 2 (SSTR2) are found in these tissues. Within the pancreas, SSTR2 are found nearly exclusively on glucagon-secreting α-cells in rodents (16,17). In humans as well, somatostatin exerts its inhibitory effect on glucagon secretion via SSTR2 found on α-cells (18,19). In the adrenal gland, SSTR2 have been widely identified in the adrenal medulla of animals and humans (20,21). It has been shown that somatostatin inhibits acetylcholine-stimulated release of epinephrine from the adrenal medulla (22,23), and this is the mechanism whereby epinephrine is released during hypoglycemia (24). SSTR2 are also found in the hypothalamus of the brain (25,26) where somatostatin also has an inhibitory effect on hormones involved in hypoglycemic counterregulation.
In isolated islets and in perifused isolated islets, the somatostatin receptor type 2 (SSTR2)-selective antagonist, DC-41-33, also known as PRL2903 dose-dependently increases glucagon secretion to an arginine stimulus, and subsequently adding somatostatin dose-dependently reverses the actions of the SSTR2 antagonist (27;28). In isolated, perfused pancreas of non-diabetic rats, this antagonist enhances glucagon secretion without affecting insulin secretion (28). It is also able to reverse the inhibitory effect of glucose-dependent insulinotropic polypeptides, GIP and GIP-(1-30)NH2, and glucagon-like polypeptide, GLP-1(7-36)NH2, on pentagastrin-stimulated gastric acid secretion in non-diabetic rats (29).
Previous experiments (28) showed the effect of the SSTR antagonist in isolated islets and pancreas (in vitro and ex vivo) but not in vivo. The effect of any SSTR antagonist has never been tested in diabetic animals. Since the glucagon response to a variety of stresses is normal in diabetic animals, including humans, and the defect is only noted during insulin-induced hypoglycemia in animals, including humans, it is essential to test the effect of any somatostatin, or somatostatin receptor, antagonist in animal models of type 1 diabetes and in diabetic humans.
Somatostatin receptor antagonists are described in U.S. Pat. No. 4,508,711 (April 1985, Coy et al.) and in U.S. Pat. No. 5,846,934 (December 1998, Bass et al.). They showed that these antagonists can increase the release of growth hormone, insulin, and glucagon. These antagonists were never tested in diabetic animals and humans, and it was not known whether these antagonists are effective during hypoglycemia when glucagon release is markedly decreased because of the enhanced sensitivity to β-cells to insulin.
Somatostatin also inhibits the secretions of corticotrophin-releasing hormone (CRH) and adrenocorticotrophic hormone (ACTH), and cortisol (i.e. hypothalamo-pituitary-adrenal (HPA) function) (30). Thus, it is of clinical interest to investigate the effect of SSTR antagonists on counterregulatory HPA hormone responses during hypoglycemia. The question of whether SSTR antagonists can improve or normalize the response of glucocorticoids to hypoglycemia or other stresses has never been investigated before. The present inventors have previously shown that carbachol, an analog of acetylcholine, injected into the third ventricle (icv) of dogs (a model of stress) increases the release of cortisol (2). However, when somatostatin was infused icv concurrently with carbachol, the cortisol responses were abolished in both non-diabetic and diabetic dogs (2;31). Therefore, a SSTR antagonist could enhance the release of cortisol also through a central mechanism and provide a mechanism whereby an SSTR2 antagonist also markedly increased the corticosterone response to hypoglycemia in diabetic rats. An additional possibility is an enhancement of corticosterone through SSTRs in the adrenal cortex, although literature has yet to report SSTR in the corticosterone synthesizing fasciculata and reticularis zonae of the adrenal cortex (32-35).
Since the α-cell is excessively sensitive to insulin in diabetic animals and humans, the key question is whether in an animal model of type 1 diabetes a somatostatin or SSTR antagonist can increase glucagon release. Hypoglycemia is the main limiting factor of intensive insulin treatment. A pharmaceutical approach which could decrease the danger of hypoglycemia would improve glycemic control in diabetic patients and could thus diminish the risk of other complications of diabetes.
Most type 1 diabetic patients suffer from frequent episodes of low blood glucose. This problem is exaggerated with tight control of blood glucose induced by frequent insulin administration. Tight control of blood glucose is necessary to minimize the danger of life-threatening diabetic complications. The danger of hypoglycemia, however, limits the possibility of desired tight control.